1. Field of the Invention
This invention relates to polishing of substrates, and more particularly to retaining ring apparatus for retaining a substrate during polishing. Specifically, this invention relates to retaining rings having surface characteristics for improving polishing uniformity and/or removal rate of a chemical mechanical polishing ("CMP") system.
2. Description of Related Art
Thin substrates, such as silicon substrates or wafers used in semiconductor fabrication, may be polished or planarized using a CMP system. Typical CMP systems function with a substrate disposed between a carrier or pressure plate and a polishing pad supported by a rotatable polishing table or platen. A circular-shaped retaining ring is typically attached to the carrier plate. During polishing, a substrate (such as semiconductor wafer) is disposed and contained between the carrier plate and polishing pad by the retaining ring. To affect polishing, the carrier plate and/or the polishing pad are brought into close proximity and moved relative to one another to impart a polishing motion to the substrate The polishing pad and/or carrier plate (including the substrate) are typically rotated at differential velocities to cause relative motion between the polishing pad and the substrate surface. Lateral, or side to side motion between the polishing pad and substrate surface may also be imparted. An abrasive slurry, such as a colloidal silica slurry may be provided between the polishing pad and substrate to facilitate polishing.
A carrier plate and retaining ring of a CMP system are typical parts of a head assembly of a CMP apparatus. The carrier plate may include vacuum ports or other mechanism for retaining a substrate against the carrier plate surface. A retaining ring is provided to prevent the substrate from being dislodged from the carrier plate during polishing. In one traditional design, a retaining ring is fixedly attached to a carrier plate and dimensioned such that the pad-side surface of the retaining ring does not contact a polishing pad surface during polishing. Instead the substrate extends beyond the surface of the retaining ring and contacts the polishing pad prior to contact with the retaining ring, thus allowing polishing of the substrate surface to occur. Because the substrate extends beyond the retaining ring during polishing, the substrate may sometimes slip out through the gap existing between the retaining ring and the polishing pad.
To reduce the potential for substrate slippage and improve edge uniformity, CMP head assemblies having an independently acting retaining ring and carrier or pressure plate have been developed. Such a system is described in U.S. Pat. No. 5,681,215, which is incorporated herein by reference. In such systems, a retaining ring and associated carrier ring are configured to independently move toward or away from a polishing pad surface, and in relation to each other. In this way, a retaining ring may be adjusted to a position where it contacts the polishing pad during the polishing process, thus reducing the potential for substrate slippage. Using such a system, a head assembly and accompanying substrate may be brought into close proximity with a polishing pad, with the retaining ring adjusted such that it contacts the polishing pad prior to the substrate. Once the retaining ring contacts the polishing pad, the carrier plate may be independently adjusted relative to the polishing pad so as to create a desired amount of pressure between the substrate lower surface (or surface to be polished) and the polishing pad surface.
One disadvantage encountered with conventional CMP systems is lack of substrate edge profile control during the CMP process. Polishing slurry distribution and profile of the polishing pad under pressure play major roles in the post-CMP film thickness profile of a semiconductor substrate. Although attempts have been made to address edge profile effects due to slurry distribution and polishing pad profile, these improvements have not been sufficient to fulfill tighter requirements for new submicron device technology.
FIG. 1 illustrates a conventional CMP polishing apparatus having a rotating platen 10 supporting a polishing pad 12 which is adhered thereto. A slurry layer 14 (containing, for example, a reactive agent such as water, abrasive particles such as silicon dioxide, and a chemically reactive catalyzer such as potassium hydroxide) may be provided on the upper surface of polishing pad 12 by a slurry introduction point 16. A head assembly 11 having independent acting carrier plate 18 and retaining ring 20 is provided to contain and support substrate 22 against upper surface of polishing pad 12 for polishing. Vacuum ports 24, or other mechanism for holding substrate 22 to carrier plate 18 may be provided. As shown by the arrows in FIG. 1, platen 10 and head assembly 11 are capable of separate motion. For example, platen 10 and head assembly 11 may rotate in the same direction, but at different speeds. Furthermore, head assembly 11 may be capable of lateral or back and forth motion.
As shown in FIG. 1, retaining ring 20 acts to hold and contain substrate 22 in place relative to carrier plate 18 during the differential movement of platen 10 relative to head assembly 11. The lower (or pad side) surface of retaining ring 20 extends toward polishing pad 12 beyond the lower (or pad side) surface of substrate 22, thus contacting and depressing the upper (or polishing side) surface of polishing pad 12 around the outer periphery of substrate 22. Movable carrier plate 18 may then be adjusted independent of the retaining ring 20 to apply the desired pressure between substrate 22 and polishing pad 12.
FIG. 2 is a simplified illustration of a conventional retaining ring 20, inverted to show smooth lower (or pad side) surface 30. FIG. 3 shows the interrelation between polishing pad 12, substrate 22 and conventional smooth-surfaced retaining ring 20 during polishing using a conventional CMP process. As may be seen in FIG. 3, the upper (or polishing side) surface of pad 12 experiences pressure from both substrate 22 and retaining ring 20 resulting in pressure discontinuities at the outer peripheral edge of substrate 22 and inner peripheral edge of retaining ring 20. The combination of these effects tends to cause the upper surface of pad 12 near the outer edge of substrate 22 to be wrinkled or bowed as shown in FIG. 3, reducing the "contact area" between upper surface of pad 12 and lower (or pad side) surface 23 of substrate 22 during polishing. This phenomenon typically causes the during-polishing profile to result in over-polishing of the flat-zone area (i.e., the area near the center of the substrate), while under-polishing the edge areas 32 of substrate 22 (i.e., the edge of the substrate 22 adjacent the retaining ring 20). Such over and underpolishing results in decreased uniformity of, for example, semiconductor substrates due to thinner film at the center of substrate 22 and/or thicker film at the edge of substrate 22. Decreased uniformity adversely affects semiconductor fabrication criteria, such as performance during etch and photo processes, as well as device yield.